In operation of wind turbines sometimes situations arise with high wind speeds and high turbulence levels causing a high load of the wind turbine in particular of the rotor blades of the wind turbine. For avoidance of any damage of the wind turbine particularly of the rotor blades of the wind turbine under these terms the wind load on the wind turbine must be reduced. An existing process by which the wind load on wind turbine is reduced in high wind conditions is simply shutting down the wind turbine when the wind speed exceeds one or more threshold values. Thus a wind turbine is e.g. shut down, when the wind speed exceeds the value of about 25 m/s for about 10 minutes, when the wind speed exceeds the value of about 28 m/s for about 30 seconds or when the wind speed exceeds the value of about 32 m/s for about 1 second.
However the wind load of a wind turbine is in general not only a function of the wind speed but also of the turbulence intensity as a descriptor for turbulences. Roughly speaking the wind speed and the turbulence defines the static wind load and for a given wind speed the turbulence defines the fatigue wind load. The existing process by which the wind turbine is shut down if a defined criterion is met partially accounts for the turbulence, wherein the wind turbine is shut down if gusts reach a wind speed of about 32 m/s for about 1 second. However assessing turbulence exclusively on the basis of a single gust is as a rule a too large simplification.
Thus the existing process of the on/off regulation of wind turbines may cause during gale conditions more or less all wind turbines in a whole region to shut down due to high wind speed to reduce the wind load on these wind turbines, even though many wind turbines could have been in operation and could have maintained power output to the grid because in particular the turbulence load may be well within safe limits.
EP 0 847 496 B1 discloses a method of operating a wind turbine, in which the power of the wind turbine as well as the operating speed of the rotor is continuously reduced when a wind velocity is reached which is in danger of overloading the wind turbine. The power of the wind turbine and the operating speed are reduced in dependence on the rise in the wind velocity or incident-stream velocity. With this method a region-wide shut down of wind turbines can be avoided because the wind turbines affected by high wind are only partially reduced in power output. However, this method does not take account of the turbulences and hence not of an important parameter determining wind turbine load.
US 2007/0018457 A1 discloses a wind turbine and a method of operating a wind turbine, wherein the rotor speed and/or the generator power are reduced in response to variables exceeding predetermined values. The variables are e.g. the wind direction relative to horizontal direction of main shaft of turbine, the turbulence of the wind or any other variable sensed by one or more sensors mounted on components of the turbine.
US 2003/0127862 A1 discloses a control system for a wind power plant comprising sensor means for the detection of measurement values to be used for direct or indirect quantification of the current loading and/or stress of the turbine occurring in dependence on the local and meteorological conditions. Downstream of said detection means, an electronic signal processing system is provided, operative to the effect that the power reduction required in the optimized condition of the wind power plant will be restricted to obtain optimum economical efficiency under the current operating conditions, both in cases of winds in the range of the nominal wind velocity and in cases of high wind velocities.
US 2003/0160457 A1 discloses a method for processing and/or predicting flow data of a flowing medium, in which from values of at least one flow parameter which are successively measured on a continuous basis at one or various locations in the medium, said flow parameter being characteristic for the speed of the medium, a time series is formed and updated which is subjected to a nonlinear deterministic prediction procedure on the basis of a locally constant phase space model for generating prediction values for the respective subsequent flow parameters. A predetermined control signal is generated if the prediction values are characteristic for an impending change in the flow speed. In case of a wind power generator inter alia an anemometer continuously measures the wind speed. The wind speed or quantities derived thereof, like the deviation between a currently measured value of the wind speed and the average over time, are subject to processing for preparing a characteristic forecast value, which is compared to a predetermined reference criterion. If during comparison, a predicted wind gust is confirmed, the wind power generator is operated in the decelerated state.